1. Field of the Invention
The present invention relates to a surface emitting laser.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is a known type of surface emitting laser.
The VCSEL includes an active region interposed between two reflectors and forms a resonator in a direction perpendicular to a substrate, thereby emitting light in a direction perpendicular to the substrate.
In surface emitting lasers, the control of transverse mode oscillation is an important issue. Considering the application of a surface emitting laser in electrophotography, telecommunication, or the like, it is desirable that the surface emitting laser operate in the single transverse mode.
To this end, a current confinement layer is formed inside the element of the surface emitting laser by selective oxidation. The current confinement layer limits a light-emitting region of an active layer and functions as a waveguide structure. Thereby, a single transverse mode operation is realized.
In the case where the single transverse mode operation of the surface emitting laser is attempted only by providing the current confinement layer, the diameter of the narrowed portion needs to be reduced. However, the small diameter of the narrowed portion results in a small light-emitting region, which makes it difficult to obtain a large laser output.
For this reason, to date, a method in which a power loss between the fundamental transverse mode and a high-order transverse mode is purposely introduced has been studied. That is, a method for obtaining a single transverse mode oscillation while maintaining a fairly large light-emitting region has been studied more than a method in which the single transverse mode operation is realized only by forming the current confinement layer.
As one of those methods, Japanese Patent Laid-Open No. 2001-284722 proposes a surface emitting laser that includes a stepped structure in order to provide a difference in reflectivities between a central region and the neighboring region including the central region of a light-emitting region. Japanese Patent Laid-Open No. 2001-284722 discloses a method for suppressing high-order transverse mode oscillation without reducing light output in the fundamental transverse mode by forming such a stepped structure and controlling the reflectivity of the neighboring region to be relatively lower than that of the central region.
Hereinafter, such a stepped structure is referred to as a surface-relief structure. It is desired that a lasing threshold current be further reduced in the surface emitting laser disclosed in Japanese Patent Laid-Open No. 2001-284722. Accordingly, the present invention provides a surface emitting laser in which an increase in the lasing threshold current can be suppressed.